In this article, we will review the potential options and practical solutions for on-site evaluation of concrete strength. Concrete (compressive) strength is by far the most important property of concrete. It represents the mechanical properties of concrete; for example, the 28 days compressive strength of concrete cylinders is the key parameter in most design codes (ACI 318-14, CSA A23.3-14). Strength is also considered (at least in the old school) a key factor for durability performance.
On-site evaluation of concrete strength is a main challenge in the condition assessment of existing infrastructure. Owners and managers of such facilities prefer non-destructive methods to avoid further damage to an already struggling structure.
1- Compression Test On Concrete Cores
Coring and testing for strength might the first and most reliable solution. In this case, concrete core is taken from the existing structure. The core needs cutting (sawing) and surface preparation. The core is tested for compressive strength.
1) This is the most reliable method to estimate the compressive strength. The method is relatively fast.
1) It is destructive. Not only it damages concrete integrity, it might affect reinforcing bars in RC structures. Rebar locating tools are needed to avoid this problem.
2) Selecting test locations can be difficult. Selecting the best location of cores is relatively subjective.
3) The locations of cores needs to be repaired.
4) Coring is not an option for owners of important structures, especially when there are concerns about further damaging the structure.
2- Pull Out Test
The concept behind this method is that the tensile force required to pull a metal disk, together with a layer of concrete, from the surface to which it is attached, is related to the compressive strength of the concrete.The pull out test is normally used for early diagnosis of strength problems. However, it can be used to evaluate the strength of concrete in existing structures. Pull out testing involves attaching a small piece of equipment to the exterior bolt, nut, screw or fixing. This is then pulled to the designated stress load level to determine how strong and secure the fixing is.
1) Relatively easy to use.
2) If relationship to strength is established, the mothod can deliver robust test results.
1) It involves crushing and damaging concrete
3- Rebound Hammer
The methods based on the rebound principle consist of measuring the rebound of a spring driven hammer mass after its impact with concrete. The test has been widely used, since its introduction in 1948. The main reason behind its popularity, is its simplicity, and convenient of use for field applications. Rebound hammer is used to evaluate the surface hardness. Malhotra (2004) argues that “there is little apparent theoretical relationship between the strength of concrete and the rebound number of the hammer. However, within limits, empirical correlations have been established between strength properties and the rebound number.”
1) It is easy to use for most field applications.
2) The test can be used to study the uniformity of concrete
1) The method is very subjective
2) surface condition, presence of rebar, presence of sub-surface voids can affect the test results
4- Ultrasonic Pulse Velocity
Ultrasonic Pulse Velocity (UPV) is an effective method for quality control of concrete materials, and detecting damages in structural components. The UPV methods have been traditionally used for the quality control of materials, mostly homogeneous materials such as metals and welded connections. With the recent advancement in transducer technology, the test has been widely accepted in testing concrete materials. The test procedure has been standardized as “Standard Test Method for Pulse Velocity through Concrete” (ASTM C 597, 2016). The concept behind the technology is measuring the travel time of acoustic waves in a medium, and correlating them to the elastic properties and density of the material. Travel time of ultrasonic waves reflects internal condition of test area. Some researchers have tried to develop a relationship between the strength and wave speed.
1) UPV can be used to detect other sub-surface deficiencies
1) The method is affected by presence of rebar, voids, and cracks.
2) There is no enough results for assessing the reliability of the method in the field.
5- Combined Methods
Combined methods involves a combination of NDT methods for predicting the on-site strength of concrete. The combination of UPV and Rebound hammer has been studied by several researcher. The combined methods often deliver more comprehensive results. The improvement of the accuracy of the strength prediction according is achieved by the use of correction factors taking into account the influence of cement type, cement content, petrologic aggregate type, fine aggregate fraction, and aggregate maximum size. The accuracy of the combination of rebound hammer and ultrasonic pulse velocity results in improved accuracy in estimating the compressive strength of concrete (Hannachi and Guetteche, 2012)